Into the end bell so rotation is counter clockwise. Left drawing. Look intently at that right brush. In it's cut form,
as shown, this brush can be in contact with one leaf of the commutator for 128.65 degrees before touching either of
the two remaining leaves. Got that? Good!
Now turn your gauze to the left brush and give it the same intense study. I show two different scenarios. Brush
cut and brush uncut. The two angles are the number of degrees of rotation that this brush will be in contact with
TWO segments WHILE the right brush is in contact with but one.
The differences are 67.64 degrees cut and 60.15 degrees uncut. As both of these numbers are greater than 60*
or 1/2 the distance between armature poles what would be your guess would happen IF timing were to exceed 60*
advance? Mr. Boggs calls out 10 - 65* timing "range" in rant.
Yep, sure would be. It would kick BACKWARDS with an effective negative timing. Why? Come on, put on your
thinking caps. Because once past 60* the pole with the star is no longer the pole being charged. Commutation has
taken place and now the one at the bottom exiting the left magnet is. It's pole reversal now seeks the opposite
magnet pulling it back into the one it just left. That's if you sneak up on it rotating the bell of a running motor.
If you have an arm like this and just flip the switch it would run backwards with an effective timing of about 55*
RETARD. Yes indeed it would be a poor motor indeed. As would be the choice in taking timing advise from
someone who doesn't grasp the geometry.
Wonder how many went running to the parts counter looking for 65* armatures the last few days. And I'm
We are not done with this drawing yet. I'll repeat part of the upper paragraph and end it differently which was my
intent before being so rudely interrupted.
The differences are 67.64 degrees cut and 60.15 degrees uncut. As both of these numbers are greater
than 60* or 1/2 the distance between armature poles what would be your guess that there is any position
where the brushes are ever in contact with ONLY two leaves at a time?
If you said zero chance, you are correct. Reason on this. Two brushes are in contact with three poles always (at
this geometry) and there is nothing practical that can be done about it.
[ note in the drawing that is appears that the upper leaf is not in contact with either brush. It is. Point contact but
in contact ]
Well doesn't that sound like "overlap"? Confused? Don't be. When I speak of overlap we are not talking about
this condition where one brush or the other is in contact with two leaves while the other is in contact with one.
We are talking about the condition where BOTH brushes are contacting two leaves each AT THE SAME TIME. If
you visualize the top left photo once more with the brushes uncut you should be able to see all on your own that at
this point in rotation both brushes would be in contact with two leaves each. That is a dead short!
Let's reason once more. Group 7 motors, by and large, employ vertical brushes. In the direction of rotation they
are but .110" tall. Less than our call out to eliminate all overlap of .0124" at this diameter. Seems Open Wing racers
have known about this performance advantage forever. So here's a brain teaser for you:
Why would these self same wing guys that migrate to drag racing NOW proclaim to the masses that the
advantage is in adding overlap? Keep your horizontal brushes they say. Helps' torque they say.
Know what? It does! It helps them maintain a torque advantage over YOU if you buy into it. Today, this is
called "telling the truth". Yes it helps torque (advantage). That's true. Just not yours.
Boys and girls we live in the era of misinformation, deception and outright lies. Sadly we also live deep enough
into these times that some actually believe the bilge! There is only one way to separate the trash from the truth. Use
the tool God gave you. Your brain. Train it with the truth and it gives back the same. Garbage in, garbage out.
Either method works. Cut brush or Vertical brushes. The reduction in overlap is an increase in "effective" dwell
and by the same number of degrees. 7.5* per pole twice per revolution. 46* per revolution. Almost 13% of a
revolution is the amount of time the motors shorted condition will be reduced (overlap) and charge time (dwell)
increased. With the amount of dyno testing I have currently under my belt I can't give you an exact figure or
percentage that this translates into in power or rpm. What I can tell you is that it does and does so without fail. Just
like planting corn seeds always produces corn plants. And just like planting corn I can't tell you HOW MUCH corn,
just that it will be corn. [with this geometry]
Note: [with this geometry] is a disclaimer for haters that can find exception to anything with an extreme enough
example. These people are like flies on a cow butt. Thank them for turning a two paragraph topic into a book.
|Kahale & Martin Research